The invention generally relates to a technique for continuous demodulation of Orthogonal Frequency Division Multiplexing (OFDM) signals.
Many recent implementations of digital wireless communication systems (wireless or cable-based systems, for example) use Orthogonal Frequency Division Multiplexing (OFDM) for environments where there are strong interference or multipath reflections. However, one disadvantage of using OFDM is the use of a Fast Fourier Transform (FFT) and an inverse FFT (IFFT) in the demodulator (for an OFDM transmitter) and modulator (for an OFDM receiver), respectively. In this manner, the calculation of the FFT and inverse FFT may add a considerable amount of complexity to the OFDM transmitter/receiver due to the large processing block that is required on each end of the communication link.
While OFDM may offer superior performance in fading, interference and multipath environments, it is not without its disadvantages. For example, one disadvantage that is associated with OFDM is the difficulty in synchronization, a difficulty that may lead to long acquisition times that may adversely effect the overall system performance. In this manner, the OFDM signal includes modulated OFDM symbols. Each symbol, in turn, appears during a particular time slot. Thus, to demodulate the OFDM signal to extract a particular symbol, the demodulation must be synchronized with the time slot. Many OFDM systems use pilot tones for channel estimation as well as to aid in the synchronization. The OFDM systems that use the pilot tones modulate or scramble the pilot tones in order to reduce the transmit peak-to-average power ratio.
For purposes of maximizing statistical multiplexing gain, many communication systems assign subsets of OFDM subcarriers to individual users, terminals or electrical devices in both the upstream and downstream directions. In this manner, the data that is associated with a particular user, terminal or electrical device is modulated at the OFDM transmitter via an associated subset of OFDM subcarriers. The resultant OFDM modulated signal is then modulated via an RF carrier signal, and this carrier modulated signal is transmitted over a wireless link (for example) for reception by an OFDM receiver. This OFDM modulation technique is commonly called OFDMA for Orthogonal Frequency Division Multiple Access.
The FFT is an N point operation, i.e., the FFT is based on a set of N subcarriers. In this manner, for the OFDM receiver, the data that is assigned to a particular subset of the subcarriers forms an FFT input vector that is processed via the FFT to produce the demodulated OFDM frequency coefficients that indicate a particular demodulated OFDM symbol.
As noted above, it is possible that some of the OFDM subcarriers may not be assigned to a particular transmitter. As a result, the block computation of the FFT for OFDM demodulation may involve calculating frequency coefficients for subcarriers that are not being used, thereby resulting in inefficient computation of the FFT.
Thus, there exists a continuing need for a technique or arrangement that addresses one or more of the problems that are stated above.